1. Field of the Invention
The present invention relates generally to the field of electrosurgery and, more particularly, to surgical devices and methods which employ very high frequency electrodes comprising an array of individual, isolated electrode terminals.
The field of electrosurgery includes a number of loosely related surgical techniques which have in common the application of electrical energy to modify the structure or integrity of patient tissue. Electrosurgical procedures usually operate through the application of very high frequency currents to cut or ablate tissue structures, where the operation can be monopolar or bipolar. Monopolar techniques rely on external grounding of the patient, where the surgical device defines only a single electrode pole. Bipolar devices comprise both electrodes for the application of current between their surfaces.
Electrosurgical procedures and techniques are particularly advantageous since they generally reduce patient bleeding and trauma associated with cutting operations. Additionally, electrosurgical ablation procedures, where tissue surfaces and volume may be reshaped, cannot be duplicated through other treatment modalities.
The use of electrosurgical procedures in electrically conductive environments, however, can be problematic. For example, many arthroscopic procedures require flushing of the region to be treated with isotonic saline (also referred to as normal saline), both to maintain an isotonic environment and to keep the field of viewing clear. The presence of saline, which is a highly conductive electrolyte, can cause shorting of the electrosurgical electrode in both monopolar and bipolar modes. Such shorting causes unnecessary heating in the treatment environment and can further cause non-specific tissue destruction.
Present electrosurgical techniques used for tissue ablation also suffer from an inability to control the depth of necrosis in the tissue being treated. Most electrosurgical devices rely on creation of an electric arc between the treating electrode and the tissue being cut or ablated to cause the desired localized heating. Such arcs, however, often create very high temperatures causing a depth of necrosis greater than 500 .mu.m, frequently greater than 800 .mu.m, and sometimes as great as 1700 .mu.m. The inability to control such depth of necrosis is a significant disadvantage in using electrosurgical techniques for tissue ablation, particularly in arthroscopic procedures for ablating and/or reshaping fibrocartilage, articular cartilage, meniscal tissue, and the like.
In an effort to overcome at least some of these limitations of electrosurgery, laser apparatus have been developed for use in arthroscopic and other procedures. Lasers do not suffer from electrical shorting in conductive environments, and certain types of lasers allow for very controlled cutting with limited depth of necrosis. Despite these advantages, laser devices suffer from their own set of deficiencies. In the first place, laser equipment can be very expensive because of the costs associated with the laser light sources. Moreover, those lasers which permit acceptable depths of necrosis (such as excimer lasers, erbium:YAG lasers, and the like) provide a very low volumetric ablation rate, which is a particular disadvantage in cutting and ablation of fibrocartilage, articular cartilage, and meniscal tissue. The holmium:YAG and Nd:YAG lasers provide much higher volumetric ablation rates, but are much less able to control depth of necrosis than are the slower laser devices. The CO.sub.2 lasers provide high rate of ablation and low depth of tissue necrosis, but cannot operate in a liquid-filled cavity.
For these reasons, it would be desirable to provide improved apparatus and methods for efficiently cutting and ablating tissue, particularly fibrocartilage, articular cartilage, meniscal tissue, and the like in arthroscopic and other procedures. Such apparatus and methods should be able to selectively cut and ablate tissue and other body structures in electrically conductive environments, particularly regions which are filled with blood, irrigated with saline, or the like. Such apparatus and methods should be able to perform cutting and ablation of tissues, particularly fibrocartilage, articular cartilage, meniscal tissue, and the like, while limiting the depth of necrosis and tissue adjacent to the treatment site. Such apparatus and methods should be amenable to precise control over the energy flux levels applied to the treatment region, and should be able to provide energy densities sufficient to provide rapid cutting and ablation. The devices should be adaptable to a wide variety of purposes, particularly including both small and large electrode surfaces, and rigid and flexible structures which can be used in open surgery, arthroscopic surgery, and other minimally invasive surgical techniques.
2. Description of the Background Art
Devices incorporating radio frequency electrodes for use in electrosurgical and electrocautery techniques are described in Rand et al. (1985) J. Arthro. Surg. 1:242-246 and U.S. Pat. Nos. 5,281,216; 4,943,290; 4,936,301; 4,593,691; 4,228,800; and 4,202,337. U.S. Pat. No. 5,281,216 describes a bipolar device having an active electrode coated with a high impedance material where the differential impedance between the active and return electrodes is optimized to provide a desired cutting effect. Vascular catheters and devices incorporating radio frequency electrodes to assist in penetrating atheroma and plaque are described in U.S. Pat. Nos. 5,281,218; 5,125,928; 5,078,717; 4,998,933; and 4,976,711, and PCT publications WO 93/20747 and WO 90/07303, the latter of which describes a catheter having four isolated electrode surfaces at its distal end. Electrosurgical power supplies including power controls and/or current limiting systems are described in U.S. Pat. No. 5,267,997 and PCT publication WO 93/20747. Surgical lasers for cutting and ablation in arthroscopic and other procedures are described in Buchelt et al. (1991) Surgery and Medicine II:271-279; and U.S. Pat. Nos. 5,147,354; 5,151,098; 5,037,421; 4,968,314; 4,785,806; 4,737,678; 4,736,743; and 4,240,441.